This invention is directed to a method for recovery of highly viscous underground hydrocarbons, particularly, a gravity stabilized thermal miscible displacement process whereby viscous hydrocarbons are mobilized by reducing the viscosity of the hydrocarbons by the application of steam and a steam-solvent mixture.
Highly viscous hydrocarbons are known to exist in subterranean formations such as the Athabasca Tar Sands in Alberta, Canada. The viscosity of these large deposits of heavy hydrocarbons, however, is so high that even after heating, conventional steam recovery methods have not proved commercially viable. Steam flooding is a well known and accepted process in the industry for recovery of viscous hydrocarbons from a formation. Generally, steam is injected into the underground formation to heat viscous hydrocarbons to reduce their viscosity sufficiently to permit the hydrocarbons to flow through the formation and into a producing well. The mobilized hydrocarbons are then pumped or flowed to the surface. Generally, the steam is injected through one well at high temperature and pressure, thereby transferring sufficient heat to the viscous hydrocarbons to lower the viscosity sufficiently to permit the hydrocarbons to flow to the producing wells. Steam flooding has been commercially successful in many of the California heavy oil deposits, but not in the more viscous reservoirs such as the Athabasca Tar Sands.
In-situ combustion has also been attempted as a method of producing highly viscous hydrocarbons with moderate success in a few applications. Like steam, however, it has not been commercially successful in very viscous deposits such as Athabasca. Recovery methods have also been proposed which call for the use of solvents, diluents, or additives, either by themselves or along with steam to further reduce the viscosity and improve fluid transmissibility within a formation.
Hydrocarbon solvents are among the additives which have frequently been proposed in the prior art for use in recovery methods for viscous hydrocarbons. The use of hydrocarbons such as aromatic solvents is within the skill of the prior art. For example, toluene and benzene are commonly used for dissolving the heavier hydrocarbon components in viscous oil, and solvents such as these can readily be vaporized for injection with steam into an underground reservoir. Upon condensing they will dissolve and dilute the viscous hydrocarbons to reduce their viscosity and improve their mobility to a greater degree than can be achieved with heat alone.
None of these prior art solvent methods, however, have been successful on a commercial basis. Some of them require injection of excessive amounts of steam and/or solvent. In others, viscous fingers of solvent, gas, steam, or other diluents, break through to the producing wells which results in the circulation of excessive amounts of the solvent, or other drive additives, thus bypassing the viscous hydrocarbons and leaving a large percentage unrecovered. These recovery methods are usually referred to as "drive" methods because an attempt is made to establish a pressure differential across the reservoir to pressure drive the viscous hydrocarbons through the formation and into the producing wells.
One of the prior art methods which attempts to avoid these problems is exemplified by the patent to Terwilliger, U.S. Pat. No. 3,608,638, which discloses a process for producing low gravity, high viscosity oils from tar sands in which pure hydrocarbon solvent vapors, such as benzene, platformate, or kerosene, are injected into the top of the tar sands at an injection well and forced through the formation to an adjacent producing well. The temperature of the injected hydrocarbons is maintained high enough to maintain a gaseous phase to establish a permeable vapor-filled channel across the top of the formation. Oil flowing into the production well is lifted through the production well at a rate to maintain a low pressure, for example, less than 100 psi, adjacent to the production well. As production continues, the upper portion of the Tar Sands is left filled with hydrocarbon vapors, or liquid of low viscosity formed by the condensation of hydrocarbon vapors, which is to be recovered by a subsequent production step.
In any oil recovery process, high production rates of heavy hydrocarbons are desirable. It is well known, however, that the flow rates of fluids through an underground reservoir or formation are proportional to the viscosity of the fluids. Accordingly, production rates of underground hydrocarbons can be increased if the viscosity can be reduced. This is particularly true for heavy hydrocarbons or hydrocarbons having high viscosity which are immobile and not recoverable when employing conventional recovery processes. Increased recovery rates have been successfully illustrated by many steam flooding processes in which the viscosity of underground hydrocarbons has been substantially reduced by heating the oil to higher temperatures by injection of steam into the reservoir. The method of the present invention, like that of the above Terwilliger patent, utilizes the technique of reduction of viscosity by temperature increase and also reduces the viscosity still further by dissolving and diluting the underground hydrocarbons with a low viscosity solvent.
Beyond this, however, the method of the present invention has several advantages over Terwilliger and other prior art solvent processes. These advantages include (1) substantially less heat and fuel requirements, (2) several fold reduction of the rate of solvent circulation, (3) attainment of higher displacement and recovery efficiencies of the heavy hydrocarbon (approaching 100%), (4) negligible solvent losses, and (5) a wider range of application of the process, including shallow depths. These advantages will be discussed in further detail.
It is one advantage of, and one essential feature of the present disclosure that the solvent is introduced into the reservoir as a vapor mixed with steam and that the solvent vapors comprise only a low percentage of the total vapor mixture. The steam/solvent vapor mixture is injected into a zone at the top of the reservoir. Since the vapor is undersaturated in solvent, only the steam condenses first and the steam provides almost all the heat required for reservoir heating. The solvent vapors pass almost completely through the hot vapor zone before condensing at the horizontal interface between the vapor and heavy hydrocarbon zones. Upon condensing, the solvent mixes with, dissolves, and dilutes the heavy hydrocarbons to reduce their viscosities to still lower values than could have been attained with heat alone. This low-viscosity solution of solvent and heavy hydrocarbons then flows downward under the force of gravity into the producing wells. Another essential feature of the present invention is that the producing wells must be open to the reservoir at some depth below the vapor zone--preferably at the bottom of the reservoir. The solvent/heavy hydrocarbon mixture is then recovered by being pumped (or more rarely, flowed) to the surface.
It is another essential feature, and an important advantage of the present invention that the pressure differential through the reservoir from injection to producing wells be controlled to very low values so that fluid flows occur almost entirely under the force of gravity alone. This results in a gravity stabilized displacement from the top of the reservoir downward. The pressure differentials are controlled to the desired low values by imposing back pressures as required against the producing wells.
Typically, prior art steam-solvent processes employ comparatively high pressure differentials from the point of injection to the point of production in the underground formation in order to increase the rate of flow of underground hydrocarbons toward the producing well. It has been well established, however, both in the laboratory and through field tests, that forced injection of low viscosity hydrocarbons into formations containing high viscosity hydrocarbons results in the formation of fingers of the low viscosity solvent breaking through at the production well. If the process is continued, a substantial portion of the injection solvent travels along these fingers or paths leaving much of the heavy hydrocarbon deposits uncontacted. Thus, while some of the objectives of a high pressure differential process may be accomplished, i.e., high production rates and high percentage recovery from the solvent swept zones, only a small portion of the hydrocarbons in the formation are affected before the process is rendered uneconomic because of the solvent bypassing effect.
The method of the present invention overcomes the disadvantages of a high pressure differential process by utilizing the force of gravity to stabilize the displacement of the heavy hydrocarbons by the steam and solvent vapor mixture. The pressure gradient across the viscous hydrocarbon deposit over most of the formation is limited to that furnished by the force of gravity. By minimizing the pressure gradient to the force of the gravity head, there is little tendency to force the light hydrocarbons through the heavy hydrocarbons and thus form low viscosity finger paths which break through at the production well.
In addition, the method of the present disclosure increases sweep efficiency by injecting hot fluids, such as steam or a steam-solvent vapor mixture, at the top of the formation and recovering heavy hydrocarbons and condensed fluids at the bottom of the formation at an adjacent production well. Since the injected fluids are hot gases, they are much lighter than the heavy hydrocarbons in the formation and therefore extend or spread across the top of the formation. The injected hot fluids remain above the underlying liquid zone until the hot gases give up their latent heat and condense to liquid and dissolve in the top layer of the underlying heavy hydrocarbons. This results in an almost horizontal solvent-steam vapor layer above the heavy hydrocarbons. The solvent-steam layer gradually moves downward as the heat of condensing steam and dilution effect of the solvent both act to reduce the viscosity of the heavy hydrocarbons to permit them to flow by gravity down to the production well. Any tendencies of the light solvent liquids to form fingers down through the colder viscous hydrocarbons, such as might be caused by local permeability variations within the formation, are counteracted by the greater hydrostatic head of the heavy hydrocarbons in the formation tending to force the lighter fluids back up to the top. The cold underlying reservoir of viscous hydrocarbons is much like an insulative barrier for the lighter fluids. Condensation of the injected solvent-steam fluids takes place along the contact area between the lighter fluids and the viscous hydrocarbons, thereby raising the temperature of the heavy hydrocarbons and increasing the mobility of the hydrocarbons. In this manner, a very stable displacement from the top to the bottom of the formation is established.
One disadvantage associated with the Terwilliger process, which uses pure solvent vapors, is that a large quantity of solvent is required to be injected into the formation. The method of the present invention, however, uses steam and solvent and adjusts the solvent to steam vapor ratio in the injected mixture so that the resulting vapor mixture is undersaturated in solvent. It is well known that at any given pressure and under such undersaturation conditions, the steam will condense first as the steam-solvent mixture gives up heat to the formation. No solvent will condense until after sufficient steam has been condensed to reduce the steam concentration to that value required for saturation at a given pressure and temperature. The steam and solvent vapors are then in equilibrium, and thereafter will both condense together.
Undersaturation of the injected mixture in solvent vapors produces several very favorable effects; first, the solvent vapors pass almost completely through the vapor zone spreading across the top of the formation before equilibrium is reached, thereby condensing at the boundary of the vapor and heavy hydrocarbon zones. Thus, use of an injected vapor mixture undersaturated with solvent vapor greatly reduces the total amount of solvent required for the disclosed recovery process without reducing the ability of the process to provide high solvent concentration in the region where it is required to contact the heavy hydrocarbons and go into solution with the hydrocarbons and thereby reduce the hydrocarbon viscosity.
Second, less heat is ultimately required with a process using a vapor mixture undersaturated in solvent. It is well known that the heat carrying capacity of hydrocarbon solvent vapors is only about one-fourth that of steam. Thus, to heat a reservoir to the same temperature, four times as much solvent must be circulated as would be needed if the heating were to be done by steam alone. The present process, in which most of the heat is provided by steam, greatly reduces the volume of hydrocarbon vapors which must be circulated, but even more importantly, it reduces the total heat requirements.
Because of the low latent heat of the solvent, it is necessary, as noted in the Terwilliger patent, that when pure solvent vapors are used, the injected vapors must be superheated in order that the hot vapor zone be maintained completely across the reservoir. The inevitable effect is that the reservoir itself is raised to a much higher temperature at the injection end than is needed to secure satisfactory producing rates. Thus, a steep temperature gradient is created across the reservoir in which the average reservoir temperature is much higher than that required with the present process which uses steam for the principal heat carrying medium and in which there is only a slight temperature gradient across the reservoir. Since the reservoir is raised to a lower average temperature in the present process, much less heat is required. As is well known, the principal expense in thermal recovery processes is the cost of the fuel which ultimately provides the reservoir heat. By reducing the heat requirements, the recovery method of the invention provides an improvement in the economics of the process.
Another advantage of injecting a steam-solvent vapor mixture undersaturated with solvent is that it provides a very high recovery efficiency from the swept zone (theoretically 100%). Once the solvent goes into solution with the heavy hydrocarbons, the solvent-heavy hydrocarbon mixture flows out of the reservoir pore spaces and down to the producing well. As is typical of all oil producing operations, both conventional and thermal recovery processes, not all of the liquid hydrocarbons can drain out of the reservoir rock. Some hydrocarbons are always trapped by the small throats in the pore spaces of the formation and cannot be recovered as a liquid. Both laboratory experiments and field tests indicate that in successful steam flood operations, the trapped unrecoverable oil, termed the irreducible saturation, generally amounts to the order of 10% to 30% of the reservoir pore space. In the method of the present disclosure, however, the heavy hydrocarbons are gradually replaced by the condensed steam-solvent liquid. The solvent concentration in the formation steadily increases with time. Thus, the final liquid trapped in the pore spaces will be essentially 100% solvent, all the oil having previously been displaced and produced.
Yet another advantage of using a vapor mixture undersaturated with solvent is that solvent losses are negligible. Unlike heavy oil, the solvent is easily distillable. As the process of the present disclosure proceeds and the horizontal condensation front drops lower into the formation, the liquid solvent trapped in the pore spaces (as described above) will be contacted by the incoming vapors of the steam-solvent mixture which is undersaturated in solvent vapor. The lean mixture vapor will rapidly reevaporate the liquid solvent trapped in the pore spaces and carry it along to the new condensation front, thereby leaving essentially no hydrocarbons or solvent in the pore spaces of the reservoir above the condensation front. At the economic end of the present process, solvent injection may be discontinued and steam alone injected into the reservoir for a few months to ensure that any solvent which was trapped in pore spaces of the reservoir is re-evaporated and recovered. This redistillation effect of the disclosed process greatly increases the ultimate heavy hydrocarbon recovery from the swept vapor zone above that which could have been obtained with steam flooding alone. It also recovers, in a continual process, the condensed solvent which would be left behind in the reservoir pore space if a pure solvent vapor or solvent liquid process were to be used.
In the Terwilliger patent, for example, it is necessary that a water drive or inert gas drive be conducted to recover the condensed solvent after all the heavy hydrocarbon has been produced. But as is well known both from laboratory experiments and field tests, these processes cannot recover all the liquid hydrocarbons trapped in the pore spaces and volumes amounting to about 10% of the pore space may be permanently lost. In the present process, however, the condensed solvent is recovered by distillation which is carried to 100% solvent recovery.
The method of the present disclosure can be operated at lower pressures and temperatures than can a steam flood which produces viscosity reduction by heat alone. By operating at lower pressures, the method can secure economic recovery from deposits which lie too close to the surface to contain the pressures required by a conventional steam flood.
The choice of solvent to be used with this method is not critical. Any light, readily distillable liquid that is miscible with the heavy hydrocarbons, will be satisfactory. Suitable solvents include, but are not limited to, gasolines, kerosene, naphthas, gas well condensates, natural gas plant liquids, intermediate refinery streams, benzene, toluene, and various distillates and cracked products.
Neither is the exact concentration of solvent critical. It may vary over a wide range from 3% solvent (by liquid volume) to as high as 65%. The method can be applied over a wide range of pressures and temperature. The operating pressure and temperature for a particular application is selected to meet the particular conditions of the reservoir to which the method is applied. The method may be operated at pressures slightly below atmospheric to as high as 1500 psi and at temperatures from 175.degree. F. to as high as 550.degree. F.